1. Field of the Invention
The present invention relates in general to a fuel injection control system for an internal combustion engine, and more particularly to a system for suppressing hunting of the engine under idling operation by controlling the fuel injection.
2. Background of the Related Art
In an internal combustion engine, it tends to occur that under a transient state, the air which has passed through a throttle valve is impeded from making a smooth flow by the interference of the volume of the intake air collecting portion and that of the intake manifold, which are located downstream o
the throttle valve, and thus the air flow fails to reach the cylinders smoothly. This phenomenon however brings about a non-negligible difference between the amount of air actually fed to the cylinders and the amount of air measured by an air-flow meter. Of course, under this condition, satisfactory air-fuel ratio control is not obtained in the fuel-injection control system.
In view of such an undesired phenomenon, a measure has been proposed which is used in an air-fuel ratio control system disclosed in Japanese Patent First Provisional Publication 4-224255.
In the system of this publication, a basic fuel injection amount "Tp" (viz., pulse width corresponding to a basic amount of fuel to be injected), calculated on the basis of both the amount of intake air measured by an air-flow meter and the engine speed, is subjected to a weighting process to derive a substantial fuel injection amount which corresponds to the amount of air actually fed to the cylinders.
Furthermore, in the system of the publication, a correction is made to the substantial fuel injection amount in accordance with a change of the same which appears in a time for which a crankshaft of the engine rotates twice or so.
However, due to its inherent construction, the system of the publication tends to have a drawback when applied to internal combustion engines of a type employing a multi-point injection system (viz., MPI) and having an intake manifold of larger volume. As is known, the multi-point injection system is the system in which a fuel injection valve is provided for each cylinder. In the engines of such type, usage of the weighting process for flattening the characteristic of the air-fuel ratio tends to increase the degree of the undesired hunting of the engine at idling.
In a system wherein the weighting process is not employed, under a transient state, the basic fuel injection amount "Tp" fails to be set to a degree corresponding to the amount of air actually fed to the cylinders. That is, during acceleration of the vehicle, the basic fuel injection amount "Tp" is set to a degree greater than the degree which corresponds to the amount of air actually fed to the cylinders. In this case, the fluctuation of the air-fuel ratio is increased. However, when the engine speed drops, the air-fuel ratio is shifted to a richer side, and when the engine speed increases, the air-fuel ratio is shifted to a leaner side, so that the undesired hunting of the engine is suppressed.
In the afore-mentioned system, wherein the weighting process is employed, the fluctuation of the air-fuel ratio can be controlled to be small. However, the hunting suppressing effect is lowered.
Accordingly, hitherto, in the internal combustion engines of the above-mentioned type which employs the multi-point injection system (MPI system) and has the larger intake manifold volume, a measure has been employed in which when a relatively large weighting is needed for setting the substantial fuel injection amount, the weighting degree for a previous degree is set small as compared with a degree at non-idling state in order to obtain the hunting suppressing effect at idling state. With this measure, the hunting suppressing effect and the air-fuel ratio fluctuation reducing effect are both obtained.
As is known, in order to effectively suppress the hunting it is necessary to instantly increase the torque of the engine just at the time when the engine speed drops. However, in the conventional air-fuel ratio control system of the above-mentioned type, the instant increase of the engine torque has not been achieved due to their inherent constructions. This will be seen from FIG. 3 of the accompanying drawings. Particularly, the hunting suppressing effect needed under engine idling after the warming up has been unsatisfied.